Iron Catalyst For Ammonia Synthesis Research Articles

Wustite phase transformations in iron catalysts for ammonia synthesis

The behaviour of wustite (ferrous oxide) during the catalyst preparation and its activation process has been studied. The Mössbauer spectroscopy and X-ray diffraction have been used to follow wustite transformations. Inside the wustite phase two different wustite structures were found – one attributed to the nonstoichiometric wustite and the second one, to the compound close to the “stoichiometric” FeO. The relative contents of both wustites depended on the cooling rate of the melted precursor. At the initial stage of activation, at low temperatures, nonstoichiometric ferrous oxide was transformed into magnetite and a compound closer to the stoichiometric FeO. The last one decomposes further into magnetite and metallic iron.

Existent state of ZrO 2 in fused iron catalyst and its influence on the reactivity of the catalyst

XRD, LRS, thermogravimetric technique, BET surface area determination and catalytic activity measurements have been used to investigate the existent state of ZrO 2 in the fused iron catalyst with and without Y 2O 3 and the influence of them on the reactivity of fused iron catalyst for ammonia synthesis. The results of XRD and LRS show that zirconia exists in the form of yttria-stabilized cubic ZrO 2 in the fused iron catalysts with Y 2O 3 We also found that for the Y 2O 3-free fused iron catalysts, cubic phase zirconia can also be stabilized by incorporating Fe 2+ or Fe 3+ cations into its lattice, and Fe 2+-stabilized zirconia is more stable than Fe 3+-stabilized zirconia. The results of a reduction experiment indicate that Y 2O 3 has a promoting effect on the reduction of ZrO 2-promoted fused iron catalyst. The catalyst containing Y 2O 3 and ZrO 2 exhibits a higher activity than a current industrial catalyst. The role of zirconia and yttria was also discussed in this work.

The micromorphology of the activated iron catalyst used for ammonia synthesis

The internal microstructure of an industrial iron catalyst for ammonia synthesis was investigated using surfaces of electropolished grains of the catalyst which had operated for 240 h in ammonia synthesis under atmospheric pressure. Optical microscopy and SEM/EDX were used to investigate the disposition of several structural motives. The heterogeneity of distribution of the promoter oxides was confirmed to exist on all levels of resolution. The microstructural discrimination of the iron metal into several distinct features and the porous structure of the iron crystals were made visible. An EXAFS experiment on these catalyst particles confirms that all iron structures are made-up from bcc iron unit cells and that the discrimination of the iron species should result from textural rather than from atomic structural differences.

A new approach to kinetic study of wet atmosphere activation of fused iron catalyst

A new experimental method has been proposed enabling the determination of the effect of reduction degree ( R) and concentration of water vapour ( c H 2O ) on the activation rate ( r= dR dt ) of the iron catalyst for ammonia synthesis. A wetless mixture of H 2 and N 2 (3:1) was introduced into the reaction chamber of an automated flow thermobalance. The applied reactor can be considered as a reactor with ideal mixing. A monolayer of catalyst grains was reduced in wet atmosphere produced by reduction itself. The reduction degree was calculated from the loss of mass recorded during the reduction. The measured reduction rate and a known fixed value of the flow rate of the gas mixture enabled us to calculate the concentration of water vapour. The experiments differ from each other by the initial mass of the catalyst and by the flow rate. The variability of these factors resulted in the variation of c H 2O within the industrially important range of 1,000–10,000 ppm. The final reduction degree varied from 78% to 95%. The results of the series of experiments performed at ca. 550°C were presented in the form of a 3-D diagram: r= r( R, c H 2O ). The diagram cut by planes at c H 2O =const yields curves r= r( R). These curves were described by a Gauss-like function. The diagram cut by plane R=10% yields the initial rate curve r= r( c H 2O ). The curve was described by an exponential function. The model function was proposed in order to describe the 3-D diagram, too. The acquisition of data by the proposed method is relatively easy, especially because there is no need, similarly as in the industrial installations, to saturate gases with water vapour.

Morphology and Crystallographic Relationships in Reduced Magnetite: A Comprehensive Structural Study of the Porous Iron Ammonia Synthesis Catalyst

Ammonia synthesis catalyst has been examined by optical microscopy and transmission electron microscopy. In reduced magnetite grains four different pore morphologies with characteristic crystallographic orientations exist. These structures are several micrometers in size and may be described as porous single crystals of bcc-iron. The dominant structure (roughly 80% of the catalyst volume) contains a nearly random network of pores and the crystallographic orientation of the porous iron is the same as that of the original magnetite. Another structure (ca. 5% by volume) has sheets of iron along the three {100} planes of the former magnetite and a 〈100〉 axis of iron normal to the sheets. A third structure (ca. 3%) with weakly sheet-like pores along the twelve {112} planes of the original magnetite has a 〈221〉 direction of iron normal to the pores. The most complex structure (very rare in the industrial catalyst) displays strongly sheet-like pores along the four {111} planes of magnetite and has a 〈100〉 axis of iron normal to the sheets. The particular orientation relations between the original magnetite and the resulting iron suggests that epitaxy plays an important role during reduction. Alumina, which is added as a promoter, reduces the lattice parameter of magnetite. The local Al3+concentration in the magnetite may influence which structure occurs by changing the likelihood of the possible epitaxy relations. Differences in activity among the various structures may exist. However, activity measurements are not a part of this structural study of the porous iron ammonia synthesis catalyst.

Relationship between Precursor Phase Composition and Performance of Catalyst for Ammonia Synthesis

The relationships among catalytic activity, reduction performance, and phase composition in precursor of melten-iron catalyst for ammonia synthesis are investigated with high-pressure testing equipment, thermogravity, and XRD. It is found that the monophase of the iron oxide in precursor is an essential condition for the high activity and easy reduction of the iron catalyst for ammonia synthesis. The activity of the catalyst is consistent with its reduction performance, both of which have similar regularity change to phase composition ratios in the precursor. The easier the catalyst is reduced, the higher the activity. FeO-based catalyst with wustite phase structure has the highest activity and the easiest reduction of all the melten-iron catalysts for ammonia synthesis. The experimental results can be explained by phase composition ratio f, which was determined by the authors.

Development of novel low temperature and low pressure ammonia synthesis catalyst

A novel catalyst based on Fe 1− x O (wustite) has been developed for ammonia synthesis. The technology has been without precedent. Its features are as follows: among all the iron catalysts for ammonia synthesis including Fe Co catalyst, the catalyst has the highest activity, the easiest reduction and stronger mechanical strength. The cost is much lower than that of the catalyst containing cobalt. The industrial use of the catalyst indicated that it has remarkable effect of saving energy. The performance figures have reached the world leading level and the catalyst has been widely used in industry in China.

In situ X-ray powder diffraction analysis of the microstructure of activated iron catalysts for ammonia synthesis

The formation of the iron species catalysing the synthesis of ammonia was followed by in situ X-ray powder diffraction. It was shown that a complex sequence of reactions during the activation process (reduction of the magnetite precursor) leads to a metastable form of α-iron. A detailed examination of the diffraction lines of iron helped to elucidate the question, why “ammonia iron” is so much superior to “normal iron”. Unusual line profiles were observed which could be controlled by changing the reaction conditions. Platelet crystallites of defective alpha-iron form the outer shell of isotropic bulk iron particles which represent the main body of the catalyst material. A shoulder at higher d values of the Fe(110) reflection points to the formation of an iron/nitrogen phase during catalytic action. The crystallographic observations are in line with previous data on paracrystallinity, but their interpretation is of a different nature. The structural effects of oxygen poisoning were investigated. It is shown, why the poisoning is detrimental for the catalytic performance and how the speculation could arise that this effect may be beneficial for the reaction.

ChemInform Abstract: Chlorine as a Poison of the Fused Iron Catalyst for Ammonia Synthesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Structure of the Activated Iron Catalyst for Ammonia Synthesis

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chlorine as a poison of the fused iron catalyst for ammonia synthesis

The poisoning effect of chlorine on the fused iron catalyst used for ammonia synthesis was studied. Chlorine adsorbs on the catalyst surface, reacting with potassium contained in the catalyst as well as with iron. The chlorine adsorption ability of the passivated catalyst is lower. The poisoning effect of chlorine on the activity of the iron catalyst is very strong and it is connected with the increase in the apparent activation energy of the ammonia synthesis process. Partial regeneration of the catalyst is possible by overheating at 920 K. The decrease of the amount of chlorine after overheating is only 12%. It suggests that chlorine is bonded not only as chlorides, but also as less volatile, oxygenchlorine compounds.

The effect of cobalt on the activation of fused iron catalyst for ammonia synthesis

AbstractBET, scanning electron microscopy, X‐ray diffraction, Mössbauer spectroscopy, X‐ray fluorescence spectroscopy and McBain thermobalance were used to investigate the effect of cobalt on the reduction behavior and activity of used iron catalyst for ammonia synthesis. Activity tests were carried out under 10 MPa in the 350–450°C temperature rang. Studies were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approx. 5.5% wt Co. Cobalt changed the reduction behavior of the catalyst. The rate of the surface change during reduction was higher for the case of the ‘cobalt catalyst’; however the rate of mass change was higher for a typical iron catalyst. The process of reduction was probably followed by the formation of an Fe3Co compound and by the surface faceting, with the exposure of an Fe(111) plane.

The effect of cobalt on the reactants adsorption and activity of fused iron catalyst for ammonia synthesis

Modern adsorption study facilities as well as scanning electron microscopy, X-ray diffraction, Mossbauer spectroscopy and X-ray fluorescence spectroscopy were used to investigate the effect of cobalt on the adsorption of nitrogen, hydrogen and ammonia on the surface of cobalt modified iron fused catalysts. Adsorption studies were carried out at the temperature range specific for ammonia synthesis (385‡C). Activity tests were carried out under 10 MPa in the 350–450‡ C temperature range. Investigations were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Introduction of cobalt changed considerably the sample behaviour during activation and ammonia synthesis. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approximately 5.5 wt% Co. Cobalt changed the adsorption behaviour of the catalyst. Chemisorption of nitrogen is much higher for “cobalt” catalysts. Growth of nitrogen chemisorption and decrease of ammonia adsorption resulted in the growth of catalytic activity of “cobalt” catalysts in ammonia synthesis.

Effect of preliminary heating on the activation of model iron catalyst for ammonia synthesis.: I. Mössbauer studies of the catalyst reduction

The oxidized form of a model, fused iron catalyst for ammonia synthesis, containing large amounts of wustite (ca. 40 wt.-%), was activated by reduction. The reduction of catalyst samples with different thermal history and different grain size was continuously monitored by thermogravimetry as well as periodically by Mössbauer spectrometry. Prior to the reduction, the samples were annealed in nitrogen atmosphere. The longer the time of primary annealing was, the faster was the reduction process. Some conclusions could be drawn about the nature of the acceleration—the induction period seemed to be shorter, whereas the reduction mechanism seemed to remain unchanged. The wustite phase disproportionates during the initial annealing, forming metallic iron and magnetite as the final products. Magnetite orginating from the decomposition of wustite showed a faster reduction rate than the primary magnetite.

Temperature-Programmed Adsorption and Desorption of Nitrogen on Iron Ammonia Synthesis Catalysts

Iron catalysts with and without a potassium promoter have been studied in a combined temperature-programmed reaction and flow microreactor system connected to a mass spectrometer. This arrangement has the advantage that sample characterization through temperature-programmed methods can be carried out alternating with measurements of synthesis reaction rates. The desorption studies demonstrate a significant shift of the N 2 desorption peak to lower temperatures with the addition of K to the catalyst. This change suggests a destabilization of dissociated nitrogen. The N 2 adsorption results are generally in good agreement with earlier studies of singly promoted catalysts. Thus the rate was found to be slow and activated. The addition of K has little effect on the temperature-programmed adsorption (TPA) curves, while for multiply promoted catalysts there is a significant shift of the main adsorption dip to lower temperatures. However, the adsorption was still activated even for this type of catalyst.

ChemInform Abstract: Effect of Cobalt on the Morphology and Activity of Fused Iron Catalyst for Ammonia Synthesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

On the structure of the activated iron catalyst for ammonia synthesis

The contribution focuses on a brief account of the activation of iron oxide precursors to ammonia synthesis catalysts and discusses the question as to why “normal iron” is so inferior in catalytic performance compared to “ammonia iron”. The combination of microstructural analysis with kinetic observations allows the conclusion that the microtexture of ammonia iron is with a platelet morphology different from isotropic iron. The role of all additives in the generation of the platelet form is examined. The presence of oxygen, which was speculated to be beneficial for the reaction under certain conditions, was found to be detrimental for the catalytic performance under the present testing conditions.

Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis

Iron catalysts with and without potassium promoter were studied in a combined temperature programmed reaction and flow micro reactor system connected to a mass spectrometer. The temperature programmed desorption (TPD) studies demonstrated a significant shift of the N2 peak to lower temperatures with the addition of K to the catalyst. The isobars suggested that this change can be interpreted as a destabilization of N-*. The N2 adsorption results were in good agreement with earlier studies of singly promoted catalysts. Thus the rate was found to be slow and activated. The rate constants were not in any disagreement with transition state theory. The addition of K had very little effect on the temperature programmed adsorption (TPA) curves, while for multiply promoted catalysts there was a significant shift to lower temperatures of the main adsorption dip. A microkinetic model for the NH3 synthesis reaction based on isobars for H2 and N2 and on the TPD results for N2 displayed a good agreement with many of the features of the experimental data. An interesting example is that the destabilization of N-* and NH-* by K can explain the difference in the promoting effect at high and low pressures. However, the conclusion is that a simple Langmuir type of kinetic model cannot result in a perfect fit of a reasonably extended experimental set of data.

Effect of cobalt on the morphology and activity of fused iron catalyst for ammonia synthesis

Abstract BET, scanning electron microscopy, X-ray diffraction, Mossbauer spectroscopy and X-ray fluorescence spectroscopy were used to investigate the effect of cobalt on the morphology and activity of fused iron catalyst for ammonia synthesis. Activity tests were carried out under 10 MPa in the 350–450°C temperature range. Investigations were performed on the traditional multipromoted iron catalyst and on a series of catalysts prepared with addition of cobalt. Introduction of cobalt increased the activity of the iron catalyst. The maximum effect was observed for 5.5 wt.-% Co. Moreover, activating action was stronger at lower temperatures. Addition of cobalt caused a change in catalysts structure. Co 2+ ions were incorporated in the crystal lattice of wustite and magnetite. They changed the surface structure and the relation Fe 2+ /Fe 3+ , however the relation of bivalent to threevalent metal ions was not affected. The change of the catalyst structure in the unreduced form produced a catalyst that possessed other surface properties. The most active catalyst adsorbed less nitrogen than those without cobalt.

Kinetics of activation of the industrial and model fused iron catalysts for ammonia synthesis

Abstract The paper summarizes our results published for many years and also adds new information. Kinetic data concerning the reduction of the oxidized forms of model as well as industrial catalysts used in ammonia synthesis were obtained in dry and in wet atmosphere containing 1% water vapour. The data for the industrial catalyst have been reassessed using three kinetic models. Modifications applied to the classical Seth-Ross model of the shrinking-core type resulted in the best fitting of this equation to the experimental data. The failure of the crackling core model to describe the kinetic data in a quantitative way is tentatively explained. The reduction of model catalysts containing enhanced amounts of wustite and/or potassium proceeds initially linearly with time. The effect of promoters, and especially of potassium, is discussed in more detail. The magnetite-alumina subsystem is responsible for the retardation effect of the water vapour on the reduction rate. A hypothesis is formulated that — in the presence of wustite or potassium — the inhibitive, Al-rich, hydrated surface layer is not effective in hindering the progress of the reduction process.